1. Field of the Invention
The present invention relates to a parallel code transmission method and an apparatus thereof. In particular, the present invention relates to a transmission line code processing method for transmitting signals between boards in a computer or a transmission processing unit or between two distant units.
2. Description of the Related Art
Conventionally, as computer techniques, signals are exchanged between internal boards of a computer and between external units in parallel. These signals are transmitted with pair cables and/or coaxial cables. However, the pair cables and coaxial cables have limitations with respect to bit rate and distance. In addition, it is known that skewing involved in a parallel data transmission (deviation of relative delay between channels) becomes a characteristic problem as the distance of the cables and the speed of data transmission increase. Thus, in consideration of the bandwidth and the transmission loss of the pair cables and coaxial cables, the maximum transmission bit rate and the maximum transmission distance thereof are at most of the orders of several 10 Mb/sec and 10 m, respectively. In other words, the use of these cables has been limited to applications of short distance and low bit rates. In addition, since the size, thickness, and weight of these cables are not suitable for applications of long distance, there are many problems to solve.
On the other hand, in recent years, as the processing speeds of the processors increase, those of computers proportionally increase. Thus, the processing speeds of signals exchanged between internal boards of a computer and between external units increase. In addition, from a flexibility point of view of the unit layout, the transmission distance of signals is becoming longer.
Moreover, with respect to transmission processing units which transmit these signals, the current mainstream of sound transmission services of the conventional public telephone networks is being replaced with broad-band integrated services digital networks (B-ISDN) which will provide full motion video transmission services requiring 1000 times wider transmission bandwidth. These B-ISDN technologies have been intensively studied worldwide. In these developing technologies, demands of high processing speeds of signal interfaces between internal boards and between external units and long distant transmission are becoming strong year after the year.
From the above-mentioned situation, in the field of computers and transmission processing units, there are demands for accomplishing interfaces which can transmit a large number of high speed signals for a long distance without deterioration of these signals. For these demands, optical parallel transmission techniques using optical fiber cables have been studied.
As so-called optical parallel transmission codes for transmitting parallel codes in optical level, for example, 4B6B code is presented in "Fundamental Study of Optical Parallel Transmission System", Document No. 2408, 70-th Anniversary National Conference of Institute of Electronics, Information and Communication Engineers (1987). This document describes a transmission line coding technique for encoding input data and transmitting the encoded data in parallel in optical level without a multiplexing process. In other words, this document does not mention techniques for simplifying the overall construction of the optical parallel link and for reducing the power consumption thereof. The simplification of the construction of the transmission line coding processing portion and the reduction of power consumption thereof are becoming important matters to solve in the conventional transmission systems as well as the optical parallel transmission system.
Generally, in the coding format "m+f" of the parallel-serial transformation (where m is the number of inputs, and f is "1" additional code), when codes are transformed with one parallel-serial transforming circuit, as the value m increases, the number of serial data increases. Thus, the transmission line speed increases. As a result, the circuit cannot be accomplished easily. On the other hand, when the value m is small, since the transmission line speed rise ratio given by (m+1) / m increases, the transmission efficiency decreases. Moreover, when codes are transformed with a plurality of parallel-serial transforming circuits, the value m which is input to each parallel-serial transforming circuit decreases. Thus, the transmission line speed rise ratio given by (m+1) / m increases.